1. Field of the Invention
The present invention relates to improvement of the material for forming reinforced metal matrix composites and the method of manufacturing the same, and more particularly forming the mixture of a reinforcement such as ceramic fibers or particles and a metal powder which permits fabrication of the metal matrix composites of improved quality and property.
2. Prior Art
It is known that a light weight metal such as aluminum, aluminum alloy, titanium, titanium alloy, magnesium, or magnesium alloy are reinforced with a light weight reinforcement having properties of high degrees of strength, elastic modulus and heat resistance such as whiskers, short fibers or particles of alumina (aluminum oxide), silicon carbide, silicon nitride or other ceramics to thereby fabricate metal matrix composites (referred to as MMC hereinbelow) material. The thus constructed composite exhibits a relatively high strength, stiffness, heat resistance and wear resistance, and thus is suitable for use in manufacturing parts of space and air crafts, an automobile car, an industrial machine or equipment and sporting goods which require light weight, high stiffness, strength and/or high heat resistance.
U.S. Pat. No. 3,833,697 discloses one of typical methods of manufacturing MMC, in which the whiskers and the metal powder are mixed with each other and the mixture is shaped by hot pressing process. The disclosed method is disadvantageous in that the property of the MMC is directly affected by the dispersed condition of whiskers in the matrix of the metal powder. The method therefore depends on the extent to which the two materials, whiskers and metal powder, are uniformly mixed with each other. With this drawback in view, the applicant of this invention has proposed in Japanese Patent Laid-Open Publication Nos.60-251922 and 62-89801 that methods and apparatus for mixing whiskers and a matrix metal powder in a uniformly distributed state, in which the ultrasonic vibration is first imparted to unravel entangled whiskers for dispersion in an organic solution, which is then stirred with the metal powder added therein to form a slurry of the whiskers and the metal particles, and the slurry is separated into the organic solution and the mixed components through processes of filtration and drying. The proposed method achieves a uniformly dispersed mixture of the whiskers and metal powder, thereby enabling fabrication of MMC of high quality.
The last-mentioned method, however, has other problems newly witnessed during the processes of filtering the slurry and drying the mixed components separated from the latter. More particularly, a lingered filtration of the slurry allows the whiskers and the metal particles to settle at different rate, thus resulting in a sedimentation or cake which has variation in whisker content in the direction of depth thereof. Further, the poor permeability and aeration of the cake elongates the time of drying if the cake of sedimentation is to be directly dried. Comminution of the cake for drying is apt to cause an objectionable segregation of the components, i.e. separation of whiskers and a metal powder due to different fluidity of components, or create a dust pollution.
There is another problem common to the conventional method of forming the mixture of a metal powder and reinforcement such as various short fibers, whiskers or various particles of ceramics or carbon. The mixture prepared in the above-described manner is often subject to segregation of the components before the mixture is used to fabricate a product such as MMC, and causes nonuniform distribution of the components and of the filling density due to its poor fluidity when the mixture is charged into the mold, with the result that the product made therefrom has only a poor quality.